I agree with what has been said about not being able to compare the two.
But the reader is put in a position to hate Umbridge more then they do Voldermort because of the first person view of Umbridge the reader gets through Harry. When Umbridge is being put forward it's directly from Harry's (the hero's) point of view: "She did this", "She said that", "Harry felt...because Umbridge". where as with Voldermort it's more second hand information, with Harry 'dreaming', articles in the news, over-heard information from adults, it isn't directly affecting Harry at the specific point in which the reader is responding to.
To give it a identifiable connection to us ask, who do you dis-like more, teachers or the government. The teachers are there, in your face, drumming shit that you don't want to know into your head. But the system wouldn't be what it is if the government wasn't the way it is, but the teachers are there, they effect the here-&-now. The imediate, direct affect is the one most people will take a disliking to most....as it is with Umbridge & Voldermort in the Order of the Phoenix.
Umbridge is doing what she believes is right, and she was right as well! If the whole wizarding community knew that Voldemort is abck, there'd be panic and chaos, and Dumbledore and the Order would have a right old time attempting to contain the madness. She had to be a strict disciplinarian inside Hogwarts for if Harry had expounded his theories any more, he may have gottn his message out, and chaos would reign, thus making Voldemorts job of genocide much easier. Plus, as a teacher, she deserves a little respect, and she was right to introduce those Educational Decrees if they brought her that deserved respect.